Deficient inhibitory control is a central feature of numerous mental disorders, including attention deficit hyperactivity disorder, substance abuse and dependence, and obsessive- compulsive disorder (OCD). As of yet, there are no established interventions broadly targeting deficits in inhibitory control, despite the relevance o this behavioral dimension to a range of disorders. Transcranial direct current stimulation (tDCS), a type of weak, noninvasive brain stimulation, has been found to improve inhibitory control in healthy individuals during laboratory tests and across behavioral training sessions when applied over the right inferior frontal gyrus (rIFG). Our long-term goal is to leverage a well-developed body of literature on the neural basis of response control and extend these findings in healthy populations in order to develop more effective treatment approaches for clinical populations with deficits in behavioral inhibition. This project will investigate a novel intervention using tDCS to enhance inhibitory control in one clinical population where problems with behavioral inhibition characterize the disorder: OCD. In the present study, 32 participants with OCD will be urn randomized to receive either active or sham tDCS over rIFG, delivered over 10 sessions in combination with exposure and response prevention (ERP). ERP is a first-line behavioral therapy for OCD which relies heavily on inhibition of OCD-related behaviors (i.e., compulsions). Given that problems with behavioral inhibition are a hallmark of OCD and that ERP relies heavily on successful inhibitory control, we expect that enhancing inhibitory control with tDCS in combination with ERP will ultimately lead to improved efficacy and/or efficiency of treatment outcomes. With this long-term objective in mind, our primary aim for this R21 Exploratory/Developmental Research Grant is to establish feasibility and acceptability of the research procedures and clinical intervention. We hypothesize that OCD patients will tolerate this combined ERP + tDCS treatment approach and will find it acceptable, as determined by rates of recruitment, treatment completion, and self-report. Our secondary aim is to gather evidence on preliminary efficacy of tDCS over rIFG as an adjunct to ERP for OCD. We hypothesize that patients in the group receiving ERP + active tDCS will show greater reduction in symptom severity over 10 sessions of the combined treatment than those receiving ERP + sham tDCS. Finally, we will aim to characterize the relationship between measures of inhibitory control and outcome of the combined intervention, using a traditional laboratory measure of inhibitory control as well as a face-valid, clinically relevant measure of OCD-related inhibitory control. For this aim, we hypothesize that response inhibition, as measured by the stop signal task, will improve more after ERP plus active vs. sham tDCS; that improvement in OCD severity will correspond to improved inhibitory control on the stop signal task; and that patients receiving ERP plus active tDCS will report less difficulty inhibiting compulsions during ERP exercises on a visual analog scale than those receiving ERP plus sham tDCS.